Fix safe-init error in Trees.scala
Fix safe-init error in Trees.scala

Fixes the following safe-init error that occurs during bootstrapping:
```
[error] -- Error: /***********/dotty/compiler/src/dotty/tools/dotc/ast/Trees.scala:815:22
[error] 815 |    assert(isEmpty || tpt != genericEmptyTree[T])
[error]     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |Call method ValDef.this.tpt.!=(dotty.tools.dotc.ast.Trees.genericEmptyTree[T]) on a value with an unknown initialization. Calling trace:
[error]     |-> @sharable val theEmptyValDef = new EmptyValDef[Type]()  [ Trees.scala:970 ]
[error]     |                                  ^^^^^^^^^^^^^^^^^^^^^^^
[error]     |-> class EmptyValDef[T >: Untyped] extends ValDef[T](  [ Trees.scala:961 ]
[error]     |   ^
[error]     |-> case class ValDef[-T >: Untyped] private[ast] (name: TermName, tpt: Tree[T], private var preRhs: LazyTree[T @uncheckedVariance])(implicit @constructorOnly src: SourceFile) [ Trees.scala:812 ]
[error]     |   ^
[error]     |-> assert(isEmpty || tpt != genericEmptyTree[T])   [ Trees.scala:815 ]
[error]     |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |-> if !assertion then scala.runtime.Scala3RunTime.assertFailed()   [ Predef.scala:10 ]
```

lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty